Most FX processors will run quality RAM at up to ~2133 MHz. without issue even though officially only 1866 MHz. is supported on the FX-8000 series. G.Skill, Corsair, Mushkin, AMD and other brands of RAM should run properly at ~2133 MHz. with the maker's specified timings for the DRAM. There is no need to pay high prices for DDR3 DRAM. You may find some DRAM that runs stable at 2400 MHz. on a FX CPU, but it would be simply hit or miss in most cases as the internal memory controller was never designed for 2400 MHz. DRAM.
The link below discusses the performance of various RAM frequencies, etc. on desktop CPU performance.